The elements responsible for the fidelity of replication (or its converse, mutability) include the replication apparatus (loosely defined to include proteins involved with DNA synthesis, nucleotide metabolism, etc.) and the DNA itself. Mutator/antimutator alleles of the various replication apparatus gene have been isolated and studied. The role of DNA is implicated by the finding that all sites within the genome are not equally mutable, as evidenced by the existence of mutational hotspots. In order to evaluate the role of gene 43 DNA polymerase in the replicative fidelity, we will examine the spectrum of spontaneous forward mutations in a small (about 300 base pairs) sequenced region of the rIIB gene. We will test the effect of various gene 43 mutator/antimutator alleles on that spectrum. It is our aim to identify the specific mutational pathways elevated or depressed by each polymerase variant. We hope to be able to correlate specific regions of DNA polymerase with specific mutational pathways. In addition we will critically examine frameshift and deletion mutation. We will insert various iterative sequences of DNA into a known site of the dispensable region of rIIB. We will examine frameshift mutation at that site as a function of the length and nucleotide composition of the insert. Next we will vary the context (the surrounding DNA sequence) of the insert to assess its effect on mutability of the insert. We will construct strains to test the hypothesis that deletions arise by semi-legitimate recombination between directly repeated sequences in the DNA. We will assay frequency of deletion formation as a function of length of the direct repeats and the extent of DNA between the direct repeats. In addition we will test the effect of mutator/antimutator alleles of the T4 replication genes on the frequency of frameshift and deletion mutation.